In manufacture of many products, such as heat exchanger tubes, for example, there is a need for large quantities of tubes cut to very precise lengths with ends accurately perpendicular to the axis of the tube stock. Machines for cutting such tubes to length have been provided heretofore. From the standpoint of economy, it is a practical necessity that such machines be capable of cutting tubes while a continuous length of tube stock is moving in the direction of its length at relatively high speed, preferably as the tubing leaves the last forming stage of a tube mill. Thus, such machines invariably employ a slide of some type on which a cut-off device is mounted and means for moving the slide at the same speed as the tube during the severing operation. Heretofore, complicated and costly devices have been employed for synchronizing the speed of the tubing and the slide on which the cut-off device is mounted. Such synchronizing devices have been not only expensive, but also incapable of functioning accurately when the tube speed is high, for example, in excess of about 175 feet per minute.
In prior art U.S. Pat. No. 4,462,290, a tube cut-off unit is arranged at the downstream end of a tube mill and is powered by a drive unit mechanically driven by the main shaft of the mill that drives the tube forming rolls. The drive unit mechanically rotates a crank which, through a link, reciprocates a slide along the path of travel of the tube. A tube cut-off blade on the slide is cam actuated by motion of the slide along its base to move in a straight line laterally of the tube to sever the tube. The slide moves at a constant speed and at the same speed as the tube when the tube is severed.
In U.S. Pat. No. 4,939,967, the cut-off machine comprises a plurality of cut-off assemblies that are reciprocated on a slide longitudinally of the moving length of tube formed by the mill, and are operated during such movement to sever plural predetermined lengths. The reciprocating and cutting movements are mechanically derived from an electric motor driven in synchronism with the speed of the tubing length. The apparatus includes mechanisms that permit the length of the severed tubes tube to be changed while the machine is operating. Each cut-off assembly includes a knife blade mounted on a chain for movement in an endless path in a plane perpendicular to the path of the tubing. A portion of the blade path is in a straight line transversely to the axis of movement of the moving length of tubing when the blade intersects the tubing path and severs the tube stock.
In U.S. Pat. No. 5,063,801, there is described an apparatus wherein a continuous length of material such as tube stock is severed into individual stock lengths by propelling the stock in a continuous uninterrupted motion in the direction of its length along a predefined path. A slide or carriage is continually reciprocated in a direction parallel to the stock path in synchronism with motion of the stock along the path such that velocity of the carriage is equal to velocity of the stock during a portion of each carriage motion cycle. A knife blade is continually rotated on the carriage about an axis parallel to the stock path and in synchronism with motion of the stock along the path such that the knife blade intersects the stock path during the portion of the carriage motion cycle in which carriage velocity equals stock velocity. As the knife blade intersects the path, the knife blade is moved radially inwardly of its axis of rotation, such that the compound result of rotary and radial motion of the knife blade with respect to its axis of rotation moves the knife blade in a straight line tangential to its axis and transverse to the path when the knife blade intersects the stock path and severe the stock.
Such cut-off machines utilize a relatively complex crank mechanism for moving the crank for reciprocating the cut off assemblies as well as an electronic circuit for controlling the servo motor insuring that the speed of movement of the cut off assemblies at cut off is synchronized with the speed of the servo motors, as described in the aforementioned U.S. Pat. No. 5,063,801.
In copending United States application Ser. No. 07/682,725 filed Apr. 9, 1991, there is disclosed a cut-off machine for cutting off predetermined lengths from a moving length of roll form material such as tubing adapted to be positioned downstream from a source such as a tube mill for receiving a moving length from the mill and severing plural predetermined lengths from the moving length. The machine comprises a plurality of cut-off assemblies which are reciprocated longitudinally of the moving length of material formed by the mill and operated during the movement to sever plural predetermined lengths. The reciprocating and cutting movements are derived from a differential beam pivot assembly wherein one end of the beam is driven in a linear reciprocating path by a crank and the other end of the beam is connected to reciprocate the cut-off assemblies such that they are driven at a speed corresponding to the speed of the tube at the time the lengths of tubing are cut off. The apparatus can be adjusted to cut tubes of different lengths by moving the pivot point of the differential beam.
Among the objectives of the present invention are to provide a tube cut-off machine wherein the cut-off assemblies are being reciprocated at a speed corresponding with the speed of movement of the tube during the time that the lengths of tubing are being cut-off; wherein the machine achieves such results mechanically; and wherein the machine functions reliably and efficiently.
In accordance with the invention, the cut-off machine includes a servo motor crank drive assembly which includes a crank having a roller which is yieldingly engaged with a cam track that has a flat tangential portion such that the roller dwells at said flat portion. During this movement, the cut-off assemblies are reciprocated driven at a speed corresponding to the speed of movement of the tube at the time the lengths of tubing are being cut-off.